I just installed Ubuntu 11.04 and I was able to use gconf-editor to change the shortcut key.

Make sure you're not running gnome-do! (If it's set to use your key combo, the Dash won't be able to bind to those keys.)

Open a terminal and run gconf-editor

Navigate to this key:

/apps/compiz-1/plugins/unityshell/screen0/options/show_launcher

Modify the show_launcher key to a single key value.

I set the value to <Super>space and now Super+space opens the dash instead of Super. Note that if you use a key combo, it seems to take a minute before you can use that to invoke the Dash. You also need to complete the key combo quickly.

If you want a more user-friendly interface, you can use the command line: